This invention relates to graded index lenses, and in particular to their use as optical fibre waveguide terminations.
FIG. 1 schematically depicts a graded index lens 10 of quarter period length, s, this; lens having a squared-off entrance facet 11 (ie. a facet perpendicular to the lens axis). Accordingly a collimated light beam 12 incident axially upon the entrance facet is brought to a focus at a plane 13 distant a behind the entrance facet 12, is reconstituted as a collimated beam at a plane 14 distant 2s behind entrance facet 12, and so on. Traces 15 schematically depict the cyclic progression between focussing and collimation.
In principle therefore, as depicted in FIG. 2, a collimated beam of light 12 can be focussed on to the end of an optical fibre 23 using a quarterperiod length (s) graded index lens 20 having entrance and exit facets 21 and 22 that are perpendicular both to the lens axis and to the incident light. To provide an adequately robust union between the fibre and the lens, the end portion of the fibre may be secured within the bore of a ferrule 24 (indicated in broken outline), typically a ceramic ferrule, and then the lens is secured to the fibre with adhesive applied between the abutting surfaces of the ferrule and the lens. The fibre 23 has an optical core of material having a bulk refractive index that is a predetermined amount greater than that of the optical cladding material with which that core is surrounded. Not all of the energy of a guided mode of such a fibre is contained within the core, and so that guided mode propagates with a velocity that Is increased in comparison with that with which light of the same wavelength will propagate through an unconfined volume of the core glass material. Accordingly the guided mode experiences an effective refractive index that is somewhat less than that of the core glass material, the amount by which it is less being determined by the extent to which the energy in the guided mode extends beyond the physical confines of the core. Typically the effective refractive index of the guided mode of the fibre 23 is not well matched with the pertinent refractive index of the graded index lens 20, and so Fresnel reflection will generally occur at the lens.fibre interface. For such reflection, the virtual image of the entrance facet is formed in a plane 25 lying a distance s behind the exit facet 22. This virtual image is therefore distant 28 from the entrance facet 21. Accordingly the Fresnel reflected light emerges after having propagated an axial distance 2s through the Ions 20. It therefore emerges as a collimated beam propagating axially in the direction directly opposite that with which the light was originally incident upon the lens. (The virtual rays are depicted by broken lines 26.) In a number of applications such retro-reflection Is undesirable or intolerable.
A known way of overcoming this retro-reflection problem is to angle the Interface between the fibre and lens as depicted in FIG. 3. In this instance the collimated beam of light 12 is Incident axially upon a graded index lens 20xe2x80x2 whose entrance facet 21xe2x80x2 is perpendicular to the lens axis, but whose exit facet 22xe2x80x2 is Inclined at an oblique angle to that axis. The end of the fibre 23xe2x80x2, and also of the surrounding ferrule 24xe2x80x2, if present, is inclined at an oblique angle to its aids. The lens 20xe2x80x2 is a quarter-period (s) long along its axis, and so the incident light is brought to a focus by the lens at the end of the fibre. If desired, the oblique angle on the end of the fibre (and its ferrule, if present) may be chosen to differ from that of the exit facet of the lens by the small amount necessary to take account of refraction at this interface. As the result of this use of a graded index lens with an obliquely oriented exit facet, any Fresnel reflection occurring at this facet is directed off-axis, and so emerges at an angle to the Incident light to be incident at a position where it can be arranged that it will do no harm.
A disadvantage of this approach to the problem of Fresnel reflection is that the preparation of angled end facets, both on the graded Index lens and on the fibre and its ferrule, adds significantly to the cost of the manufacture of a graded index lens terminated optical fibre, in some circumstances more than doubling the lost.
An object of the present invention is, in respect of the launching of a beam of collimated light into the -end of an optical fibre via a graded index lens, to reduce, without recourse to the use of angled facets on that graded index lens. the intensity of Fresnel reflected light directed back along the path of the incident light
According to a first aspect of the present invention, there is provided a two part compound lens consisting of a plane-parallel ended first lens part united in end-to-end relationship with a plane-parallel ended second lens part, the compound lens having a lens axis which is co-directional with the normal to both ends of the lens parts, wherein the first lens part is a graded index lens of length d1 and quarter period s, and the second lens part is of uniform refractive index and of length d2, wherein d1=(2nxe2x88x921)sxe2x88x92xcex94, where n is a positive integer and s greater than xcex94 greater than 0, xcex94 is a length of the first lens which is removed and substituted by the second lens part of length d2, d2 is a length possessing the property that collimated light incident axially upon that end face of the graded Index first lens part that is remote from the end facet united with the second lens part, is brought to a substantial focus at that end facet of the second lens part that is remote from the end facet united with the first lens part.
It is, however, not always necessary or desirable to provide the second lens part with parallel end facets. In certain applications there can be advantage in providing this second lens part with planar end facets that are deliberately chosen to define a small wedge angle that makes the thickness of this lens part slightly greater than d2 at one side edge thereof, and slightly greater than d2 at the opposite side
According to a first aspect of the present invention, there is provided a two-part compound lens consisting of a plane-parallel ended first lens par and a planar ended second lens part united in end-toend relationship, wherein the first lens part of the compound lens has a lens axis normal to its ends and is a graded index lens of quarter period s length and of length d1, where d1=(2nxe2x88x921)sxc2x7xcex94, and where n is a positive integer and s greater than xcex94 greater than 0, xcex94 is a length of the first lens which is removed and substituted by the second lens part of length d2 and wherein the planar ends of the second lens part of the compound lens are inclined at a small wedge angle, and are chosen to provide a lens part thickness of less than d2 at one side of the second lens part, and of greater than d2 at the other, where d2 is a length possessing the property that collimated light incident axially upon the end face of the graded Index first lens part that is remote from the end facet united with the second lens part, is brought to a substantial focus at a depth d2 into the material of the second lens part.
The invention also resides in an optical fibre terminated with a two part compound lens as set out In the three preceding paragraphs, wherein one end of the fibre abuts the second lens part of the compound lens, and has a core providing a guided mode of that fibre with an effective refractive index substantially matched with the refractive index of the second lens part of the compound lens.
It should be understood that the replacement of a simple graded index lens with a compound lens whose graded part has the same refractive index profile as that the simple graded index lens, and whose uniform refractive index part has its refractive index matched with the effective refractive index of the fibre, does not result in any reduction of the Fresnel reflection coefficient. Accordingly, the total reflected power, expressed as a proportion of the Incident power, is the same in both instances. However the plane in which the power Is reflected Is different in the two instances. The improved performance of the compound lens is therefore seen not to result from a reduction in reflected power, but from altering the position of the plane of reflection as to change the angular spread of that reflection so that less power is reflected back directly along the path of the incident light.
Other features and advantages of the invention will be readily apparent from the following (description of a preferred embodiment of the invention, from the drawings and from the claims.